Air-conditioning apparatus

ABSTRACT

An air-conditioning apparatus includes a refrigerant circuit in which a compressor, an indoor heat exchanger, a first expansion device, an outdoor heat exchanger, and a flow switching device are sequentially connected to each other; a hot gas bypass pipe coupling a discharge port of the compressor and the flow switching device to each other; and a controller. The outdoor heat exchanger includes an upper heat exchanger and a lower heat exchanger having passages in parallel with each other. The outdoor heat exchanger includes a plurality of hairpin pipes, which are part of a heat transfer pipe. When the upper heat exchanger is defrosted, all hairpin pipes that are located at a lowermost step of the upper heat exchanger are used as refrigerant inlets. When the lower heat exchanger is defrosted, all hairpin pipes that are located at an uppermost step of the lower heat exchanger are used as refrigerant inlets.

TECHNICAL FIELD

The present disclosure relates to an air-conditioning apparatus that simultaneously defrosts an outdoor heat exchanger and heats a room.

BACKGROUND ART

In the related art, some air-conditioning apparatuses simultaneously defrost an outdoor heat exchanger and heat a room (see, for example, Patent Literature 1). In Patent Literature 1, an air-conditioning apparatus includes a refrigerant circuit in which a compressor, a four-way valve, an indoor heat exchanger, a pressure reducing device, and an outdoor heat exchanger are connected to each other by refrigerant pipes, and is provided with a bypass in which hot gas flows from a discharge port of the compressor to the outdoor heat exchanger. The refrigerant circuit in the outdoor heat exchanger is divided into two portions. A lower portion is a lower heat exchanger and an upper portion is an upper heat exchanger.

The controller opens and closes main circuit opening-closing mechanisms and second expansion devices to perform a heating-defrosting operation for performing heating with the lower heat exchanger while defrosting the upper heat exchanger and then performing heating with the upper heat exchanger while defrosting the lower heat exchanger. By performing the heating-defrosting operation in this manner, the temperature in a room is less likely to decrease while a heating capacity of an indoor unit is less likely to decrease. Therefore, it is possible to prevent comfortableness in the room from being lost while performing defrosting.

In addition, in Patent Literature 1, the outdoor heat exchanger includes paths that are a plurality of refrigerant passages. A cooling-inlet-side pipe, which is one of the plurality of paths, that is, an inlet of a path at the time of the heating-defrosting operation for continuing heating while performing defrosting is disposed at a boundary between the upper heat exchanger and the lower heat exchanger.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2008-64381

SUMMARY OF INVENTION Technical Problem

In Patent Literature 1, when the upper heat exchanger is defrosted during the heating-defrosting operation, drain water melts on the upper heat exchanger and flows downward on a heat transfer fin of the outdoor heat exchanger. When the lower heat exchanger is used as an evaporator and the temperature at the heat exchanger decreases to 0 degrees C. or lower, the drain water refreezes on the lower heat exchanger. If the lower heat exchanger is subsequently defrosted, the drain water refrozen on the lower heat exchanger melts. However, as the upper heat exchanger is used as an evaporator, heat transfer between the upper heat exchanger and the lower heat exchanger makes it hard for the temperature to increase at the boundary between the upper heat exchanger and the lower heat exchanger. This causes a problem in that ice of the refrozen drain water grows without melting and disturbs an air flow at the heat transfer fin of the outdoor heat exchanger, leading to a decrease in a heating capacity, for example.

In a path configuration having been employed, only one hairpin pipe among a plurality of hairpin pipes located at the boundary between the upper heat exchanger and the lower heat exchanger is used as an inlet pipe for high-temperature gas refrigerant during the heating-defrosting operation. This causes a problem in that temperature does not increase at the other hairpin pipes located at the boundary and at the heat transfer fin around the other hairpin pipes, making it difficult to melt the ice of the refrozen drain water.

The present disclosure is made to solve the above problems and an object of the present disclosure is to provide an air-conditioning apparatus configured such that a decrease in a heating capacity owing to refreezing of the drain water at the boundary between the upper heat exchanger and the lower heat exchanger is less likely to be caused.

Solution to Problem

An air-conditioning apparatus according to an embodiment of the present disclosure includes a refrigerant circuit in which a compressor, an indoor heat exchanger, a first expansion device, an outdoor heat exchanger, and a flow switching device are sequentially connected to each other by pipes and through which refrigerant circulates; a hot gas bypass pipe coupling a discharge port of the compressor and the flow switching device to each other; and a controller. The compressor is configured to compress and discharge the refrigerant. The indoor heat exchanger is configured to allow the refrigerant to exchange heat with indoor air. The first expansion device is configured to reduce pressure of the refrigerant. The outdoor heat exchanger includes an upper heat exchanger and a lower heat exchanger having passages in parallel with each other and is configured to allow the refrigerant to exchange heat with outdoor air. The flow switching device is configured to switch flows of the refrigerant toward the upper heat exchanger and the lower heat exchanger. The controller is configured to perform a heating-defrosting operation for alternately defrosting the upper heat exchanger and the lower heat exchanger while performing a normal heating operation. The outdoor heat exchanger includes a plurality of hairpin pipes, which are part of a heat transfer pipe. When the upper heat exchanger is defrosted, among the plurality of hairpin pipes, all hairpin pipes located at a lowermost step of the upper heat exchanger are used as refrigerant inlets. When the lower heat exchanger is defrosted, among the plurality of hairpin pipes, all hairpin pipes located at an uppermost step of the lower heat exchanger are used as refrigerant inlets.

Advantageous Effects of Invention

In the air-conditioning apparatus according to an embodiment of the present disclosure, the hairpin pipes located at the lowermost step of the upper heat exchanger and the uppermost step of the lower heat exchanger, that is, all the hairpin pipes located at the boundary between the upper heat exchanger and the lower heat exchanger are used as refrigerant inlets when defrosting is performed. Thus, drain water at the boundary between the upper heat exchanger and the lower heat exchanger is less likely to refreeze and consequently a heating capacity is less likely to decrease.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of a refrigerant circuit of an air-conditioning apparatus according to Embodiment 1 of the present disclosure.

FIG. 2 is a schematic diagram illustrating a cross-section of an outdoor heat exchanger and an outdoor fan of an air-conditioning apparatus of the related art.

FIG. 3 is a schematic diagram illustrating a cross section of a portion around a boundary between an upper heat exchanger and a lower heat exchanger when the upper heat exchanger of the outdoor heat exchanger of the air-conditioning apparatus of the related art is defrosted.

FIG. 4 is a schematic diagram illustrating a cross section of the portion around the boundary between the upper heat exchanger and the lower heat exchanger when the lower heat exchanger of the outdoor heat exchanger of the air-conditioning apparatus of the related art is defrosted.

FIG. 5 is a Mollier chart at the time of a heating-defrosting operation of the air-conditioning apparatus of the related art.

FIG. 6 is a temperature distribution chart of the portion around the boundary between the upper heat exchanger and the lower heat exchanger at the time of the heating-defrosting operation of the air-conditioning apparatus of the related art.

FIG. 7 is a schematic diagram illustrating a cross section of a portion around a boundary between an upper heat exchanger and a lower heat exchanger when the upper heat exchanger of an outdoor heat exchanger of the air-conditioning apparatus according to Embodiment 1 of the present disclosure is defrosted.

FIG. 8 is a schematic diagram illustrating a cross section of the portion around the boundary between the upper heat exchanger and the lower heat exchanger when the lower heat exchanger of the outdoor heat exchanger of the air-conditioning apparatus according to Embodiment 1 of the present disclosure is defrosted.

FIG. 9 is a Mollier chart at the time of a heating-defrosting operation of the air-conditioning apparatus according to Embodiment 1 of the present disclosure.

FIG. 10 is a temperature distribution chart of the portion around the boundary in the outdoor heat exchanger at the time of the heating-defrosting operation of the air-conditioning apparatus according to Embodiment 1 of the present disclosure.

FIG. 11 is a diagram of a refrigerant circuit of an air-conditioning apparatus according to Embodiment 2 of the present disclosure.

FIG. 12 is a diagram of a refrigerant circuit of an air-conditioning apparatus according to Embodiment 3 of the present disclosure.

FIG. 13 is a diagram of a refrigerant circuit of an air-conditioning apparatus according to Embodiment 4 of the present disclosure.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to the embodiments described below. In the drawings below, the relative sizes of components may differ from actual relative sizes.

Embodiment 1

FIG. 1 is a diagram of a refrigerant circuit of an air-conditioning apparatus 100 according to Embodiment 1 of the present disclosure.

The air-conditioning apparatus 100 according to Embodiment 1 includes an outdoor unit 1 and an indoor unit 2 and is a split air-conditioning apparatus in which the outdoor unit 1 and the indoor unit 2 are connected to each other by refrigerant pipes 83 and 84 and electric wires (not illustrated).

Outdoor Unit

The outdoor unit 1 includes a compressor 10, a first flow switching device 20, a first expansion device 30, a second expansion device 60, a second flow switching device 70, an outdoor heat exchanger 50, an outdoor fan 500, an outdoor-air-temperature detection device 200, a first boundary-temperature detection device 201, a second boundary-temperature detection device 202, and a controller 300.

Indoor Unit

The indoor unit 2 includes an indoor heat exchanger 40 and an indoor fan 400.

The air-conditioning apparatus 100 has a refrigerant circuit through which refrigerant circulates. The refrigerant circuit includes the compressor 10, the first flow switching device 20, the indoor heat exchanger 40, the first expansion device 30, the outdoor heat exchanger 50, and the second flow switching device 70 that are sequentially connected to each other by refrigerant pipes 81 to 85, 86A to 87A, 86B to 87B, 89, and 91. Various refrigerants can be used as the refrigerant that circulates through this refrigerant circuit. For example, refrigerant such as R32 and R410A may be used.

In addition, a discharge port of the compressor 10 and an A port of the second flow switching device 70 are connected to each other by hot gas bypass pipes 80 and 88. The second expansion device 60 is disposed between the hot gas bypass pipes 80 and 88.

Refrigerant Pipes and Hot Gas Bypass Pipes

One end of the refrigerant pipe 81 is connected to the discharge port of the compressor 10. The other end of the refrigerant pipe 81 branches into the hot gas bypass pipe 80 and the refrigerant pipe 82. The refrigerant pipe 82 is connected to a G port of the first flow switching device 20. The hot gas bypass pipe 80 is connected to the second expansion device 60. The refrigerant pipe 83 connects an H port of the first flow switching device 20 and the indoor heat exchanger 40 to each other. The refrigerant pipe 84 connects the indoor heat exchanger 40 and the first expansion device 30 to each other. One end of the refrigerant pipe 85 is connected to the first expansion device 30. The other end of the refrigerant pipe 85 branches into the refrigerant pipe 86A and the refrigerant pipe 86B. The refrigerant pipe 86A is connected to an upper heat exchanger 50A of the outdoor heat exchanger 50. The refrigerant pipe 86B is connected to a lower heat exchanger 50B of the outdoor heat exchanger 50. The refrigerant pipe 87A connects the upper heat exchanger 50A and a B2 port of the second flow switching device 70 to each other. The refrigerant pipe 87B connects the lower heat exchanger 50B and a B1 port of the second flow switching device 70 to each other. The hot gas bypass pipe 88 connects the second expansion device 60 and the A port of the second flow switching device 70 to each other. The refrigerant pipe 89 connects a C port of the second flow switching device 70 and an E port of the first flow switching device 20 to each other. The refrigerant pipe 91 connects an F port of the first flow switching device 20 and a suction port of the compressor 10 to each other.

Controller 300

The controller 300 includes, for example, dedicated hardware, a Central Processing Unit (also referred to as a CPU, a central processing device, a processing device, an arithmetic unit, a microprocessor, or a processor) that executes a program stored in a memory, or memories such as a RAM and a ROM.

Operations of the air-conditioning apparatus 100 according to Embodiment 1 include two operations, that is, a cooling operation and a heating operation. The heating operation includes two operations, that is, a normal heating operation in which both the upper heat exchanger 50A and the lower heat exchanger 50B are used as evaporators, and a heating-defrosting operation in which one of the upper heat exchanger 50A and the lower heat exchanger 50B is used as an evaporator and the other of the upper heat exchanger 50A and the lower heat exchanger 50B is used as a condenser. The controller 300 performs one of these operations in accordance with selection made by a user, for example.

The compressor 10 is capable of changing an operation frequency in accordance with a control signal received from the controller 300. Changing the operation frequency of the compressor 10 enables adjustment of output of the compressor 10. Various compressors such as a rotary compressor, a reciprocating compressor, a scroll compressor, and a screw compressor can be used as the compressor 10.

The first flow switching device 20 is a device that switches between the cooling operation and the heating operation. The first flow switching device 20 is, for example, a four-way valve but may be a combination of two-way valves and three-way valves. In the heating operation, as indicated by a solid line in FIG. 1, the first flow switching device 20 connects the refrigerant pipe 82, which is a discharge pipe of the compressor 10, and the refrigerant pipe 83 to each other and the refrigerant pipe 89 and the refrigerant pipe 91, which is a suction pipe of the compressor, to each other. In the cooling operation, as indicated by a broken line in FIG. 1, the first flow switching device 20 connects the refrigerant pipe 82 and the refrigerant pipe 89 to each other and the refrigerant pipe 83 and the refrigerant pipe 91 to each other.

The first expansion device 30 is a device that reduces pressure of the refrigerant flowing into the first expansion device 30 and is, for example, an expansion valve.

The indoor fan 400 is disposed adjacently to the indoor heat exchanger 40 and supplies air to the indoor heat exchanger 40.

The outdoor fan 500 is disposed adjacently to the outdoor heat exchanger 50 and supplies air to the outdoor heat exchanger 50.

The outdoor heat exchanger 50 is a finned-tube heat exchanger including a heat transfer pipe, a plurality of heat transfer fins 52, and a header 53. The heat transfer pipe includes a plurality of U-shaped hairpin pipes 51 and U-bend pipes (not illustrated) connecting the hairpin pipes 51 to each other (see FIGS. 7 and 8 described later). The outdoor heat exchanger 50 is divided into two portions that are the upper heat exchanger 50A and the lower heat exchanger 50B. The upper heat exchanger 50A and the lower heat exchanger 50B are disposed vertically and are connected in parallel with each other. In the upper heat exchanger 50A and the lower heat exchanger 50B, the heat transfer fins 52 are separated. However, in the upper heat exchanger 50A and the lower heat exchanger 50B, the heat transfer fins 52 need not be separated. A passage of the upper heat exchanger 50A and a passage of the lower heat exchanger 50B are in parallel with each other. The flow direction of the refrigerant will be described in description of the operations.

The outdoor-air-temperature detection device 200 detects an outdoor air temperature. The first boundary-temperature detection device 201 is disposed at an inlet pipe of the upper heat exchanger 50A and detects an inlet temperature of the upper heat exchanger 50A. The temperature detected by this first boundary-temperature detection device 201 has a correlation with a temperature at the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A. Thus, by correcting the temperature detected by the first boundary-temperature detection device 201, the temperature at the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A can be indirectly detected. The second boundary-temperature detection device 202 is disposed at an inlet pipe of the lower heat exchanger 50B and detects an inlet temperature of the lower heat exchanger 50B. The temperature detected by this second boundary-temperature detection device 202 has a correlation with a temperature at the hairpin pipes 51 located at the uppermost step of the lower heat exchanger 50B. Thus, by correcting the temperature detected by the second boundary-temperature detection device 202, the temperature at the hairpin pipes 51 located at the uppermost step of the lower heat exchanger 50B can be indirectly detected. The outdoor-air-temperature detection device 200, the first boundary-temperature detection device 201, and the second boundary-temperature detection device 202 are, for example, thermistors. The first boundary-temperature detection device 201 may be disposed not at the inlet pipe of the upper heat exchanger 50A but at the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A. This enables direct detection of the temperature at the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A without correction of the temperature detected by the first boundary-temperature detection device 201. The second boundary-temperature detection device 202 may be disposed not at the inlet pipe of the lower heat exchanger 50B but at the hairpin pipes 51 located at the uppermost step of the lower heat exchanger 50B. This enables direct detection of the temperature at the hairpin pipes 51 located at the uppermost step of the lower heat exchanger 50B without correction of the temperature detected by the second boundary-temperature detection device 202.

The hot gas bypass pipes 80 and 88 are provided to use part of the refrigerant discharged from the compressor 10 in defrosting of (removal of frost on) the upper heat exchanger 50A and the lower heat exchanger 50B. The second expansion device 60, which is for example an expansion valve, is connected as an expansion mechanism to the hot gas bypass pipe 80. The second expansion device 60 reduces the pressure of the part of the refrigerant discharged from the compressor 10 to a medium pressure.

The refrigerant is then led through the second flow switching device 70 to a defrosting target among the upper heat exchanger 50A and the lower heat exchanger 50B.

The operations of the air-conditioning apparatus 100 according to Embodiment 1 will be described next.

Cooling Operation

The cooling operation will be described first. The compressor 10 suctions refrigerant from the refrigerant pipe 91 and compresses the refrigerant. The compressed high-temperature and high-pressure gas refrigerant is discharged from the compressor 10 and flows to the refrigerant pipe 89 through the refrigerant pipe 81, the refrigerant pipe 82, and the first flow switching device 20.

The gas refrigerant flowing through the refrigerant pipe 89 is caused to branch by the second flow switching device 70 as indicated by the solid line in FIG. 1. Part of the gas refrigerant flows from the B2 port to the refrigerant pipe 87A, and the rest of the gas refrigerant flows from the B1 port to the refrigerant pipe 87B. The gas refrigerant branching to the refrigerant pipe 87A flows through the upper heat exchanger 50A and exchanges heat with outdoor air at the upper heat exchanger 50A to condense. The resultant high-pressure liquid refrigerant flows through the refrigerant pipe 86A. The gas refrigerant branching to the refrigerant pipe 87B flows through the lower heat exchanger 50B and exchanges heat with outdoor air at the lower heat exchanger 50B to condense. The resultant high-pressure liquid refrigerant flows through the refrigerant pipe 86B.

The controller 300 is capable of adjusting, using a control signal, the rotation frequency of the outdoor fan 500. Adjustment of the rotation frequency of the outdoor fan 500 by the controller 300 changes an amount of air sent to the outdoor heat exchanger 50. Consequently, an amount of heat exchanged between the refrigerant and the air at the outdoor heat exchanger 50 can be adjusted.

The liquid refrigerant flowing through the refrigerant pipe 86A and the liquid refrigerant flowing through the refrigerant pipe 86B join together at a joint portion of the refrigerant pipes 86A and 86B and the refrigerant pipe 85 and flow through the refrigerant pipe 85. The pressure of the liquid refrigerant is reduced by the first expansion device 30, and the resultant low-temperature and low-pressure two-phase refrigerant flows to the refrigerant pipe 84. The controller 300 is capable of adjusting, using a control signal, an opening degree of the first expansion device 30. At this time, adjustment of the opening degree of the first expansion device 30 by the controller 300 enables adjustment of a reduction amount of pressure of the refrigerant. Increasing the opening degree of the first expansion device 30 causes the pressure of the refrigerant at an outlet port of the first expansion device 30 to increase and the quality of the refrigerant to decrease. In contrast, decreasing the opening degree of the first expansion device 30 causes the pressure of the refrigerant at the outlet of the first expansion device 30 to decrease and the quality of the refrigerant to increase.

The liquid refrigerant flowing through the refrigerant pipe 84 flows into the indoor heat exchanger 40 and exchanges heat with indoor air at the indoor heat exchanger 40 to evaporate. The resultant low-temperature and low-pressure gas refrigerant flows through the refrigerant pipe 83.

The controller 300 is capable of adjusting, using a control signal, the rotation frequency of the indoor fan 400. Adjustment of the rotation frequency of the indoor fan 400 by the controller 300 changes an amount of air sent to the indoor heat exchanger 40. Consequently, an amount of heat exchanged between the refrigerant and the air at the indoor heat exchanger 40 can be adjusted.

The gas refrigerant flowing through the refrigerant pipe 83 flows into the compressor 10 again from the refrigerant pipe 91 through the first flow switching device 20.

Normal Heating Operation

The normal heating operation will be described next. The compressor 10 suctions refrigerant from the refrigerant pipe 91 and compresses the refrigerant. The compressed high-temperature and high-pressure gas refrigerant is discharged from the compressor 10 and flows to the refrigerant pipe 83 through the refrigerant pipe 81, the refrigerant pipe 82, and the first flow switching device 20.

The gas refrigerant that flows from the refrigerant pipe 83 into the indoor heat exchanger 40 exchanges heat with indoor air at the indoor heat exchanger 40 to condense. The resultant high-pressure liquid refrigerant flows through the refrigerant pipe 84. At this time, adjustment of the rotation frequency of the indoor fan 400 by the controller 300 changes an amount of air sent to the indoor heat exchanger 40. Consequently, an amount of heat exchanged between the refrigerant and the air at the indoor heat exchanger 40 can be adjusted.

The liquid refrigerant that flows out from the indoor heat exchanger 40 flows through the refrigerant pipe 84. The pressure of the liquid refrigerant is reduced by the first expansion device 30, and the resultant low-temperature and low-pressure two-phase refrigerant flows to the refrigerant pipe 85. At this time, adjustment of the opening degree of the first expansion device 30 by the controller 300 enables adjustment of a reduction amount of pressure of the refrigerant. Increasing the opening degree of the first expansion device 30 causes the pressure of the refrigerant at the outlet port of the first expansion device 30 to increase and the quality of the refrigerant to decrease. In contrast, decreasing the opening degree of the first expansion device 30 causes the pressure of the refrigerant at the outlet port of the first expansion device 30 to decrease and the quality of the refrigerant to increase.

The two-phase refrigerant flowing through the refrigerant pipe 85 branches to the refrigerant pipe 86A and the refrigerant pipe 8611 The two-phase refrigerant branching to the refrigerant pipe 86A flows through the upper heat exchanger 50A and exchanges heat with outdoor air at the upper heat exchanger 50A to evaporate. The resultant low-temperature and low-pressure gas refrigerant flows through the refrigerant pipe 87A. The two-phase refrigerant branching to the refrigerant pipe 86B flows through the lower heat exchanger 50B and exchanges heat with outdoor air at the lower heat exchanger 50B to evaporate. The resultant low-temperature and low-pressure gas refrigerant flows through the refrigerant pipe 87B. At this time, adjustment of the rotation frequency of the outdoor fan 500 by the controller 300 changes an amount of air sent to the outdoor heat exchanger 50. Consequently, an amount of heat exchanged between the refrigerant and the air at the outdoor heat exchanger 50 can be adjusted.

The gas refrigerant flowing through the refrigerant pipe 87A and the gas refrigerant flowing through the refrigerant pipe 87B join together through the second flow switching device 70 as indicated by the solid line in FIG. 1 and flow from the C port to the refrigerant pipe 89. The gas refrigerant flowing through the refrigerant pipe 89 flows into the compressor 10 again from the refrigerant pipe 91 through the first flow switching device 20.

During the normal heating operation, the second expansion device 60 may be open or fully closed. The second flow switching device 70 allows the B1 port and the C port to communicate with each other and allows the B2 port and the C port to communicate with each other. Thus, even if the refrigerant is present in the hot gas bypass pipe 88, the refrigerant does not flow out from the A port to the other ports.

While the normal heating operation is performed in the above manner, the outdoor heat exchanger 50 may be frosted and the necessity of defrosting may arise. In such a case, it is conceivable to temporarily stop the normal heating operation, switch the operation to the cooling operation, and perform a defrosting operation for causing the high-temperature and high-pressure gas refrigerant compressed by the compressor 10 to flow through the outdoor heat exchanger 50. In this case, however, as the normal heating operation is interrupted, the room temperature decreases and comfortableness in the room is lost.

Heating-Defrosting Operation

The heating-defrosting operation will be described next.

In the heating-defrosting operation, while the normal heating operation is continued, the second flow switching device 70 is caused to operate such that the upper heat exchanger 50A and the lower heat exchanger 50B are alternately defrosted.

If the outdoor heat exchanger 50 is frosted while the normal heating operation is performed and the necessity of defrosting, for example, the upper heat exchanger 50A arises, the second flow switching device 70 is caused to operate such that the hot gas bypass pipe 88 and the refrigerant pipe 87A are connected to each other and the refrigerant pipe 87B and the refrigerant pipe 89 are connected to each other. Consequently, part of the high-temperature and high-pressure gas refrigerant discharged from the compressor 10 flows into the hot gas bypass pipe 80, and the rest of the high-temperature and high-pressure gas refrigerant flows to the indoor heat exchanger 40 through the refrigerant pipe 82, the first flow switching device 20, and the refrigerant pipe 83.

The pressure of the high-temperature and high-pressure gas refrigerant that has flowed into the hot gas bypass pipe 80 is reduced by the second expansion device 60. The refrigerant then flows into the upper heat exchanger 50A, which is a defrosting target, through the hot gas bypass pipe 88, the second flow switching device 70, and the refrigerant pipe 87A. The high-temperature gas refrigerant that has flowed into the upper heat exchanger 50A exchanges heat with frost to condense, and the upper heat exchanger 50A is thus defrosted.

At this time, the controller 300 changes the opening degree of the second expansion device 60 to adjust an amount of the refrigerant that flows into the upper heat exchanger 50A, which is the defrosting target, and consequently can adjust an amount of heat exchanged between the refrigerant and the frost.

Increasing the opening degree of the second expansion device 60 causes an amount of refrigerant at the outlet of the second expansion device 60 to increase and an amount of refrigerant that flows through the upper heat exchanger 50A to increase. Consequently, an amount of heat exchanged between the refrigerant and the frost increases. At this time, as an amount of refrigerant that flows through the indoor heat exchanger 40 decreases, the heating capacity decreases.

In contrast, decreasing the opening degree of the second expansion device 60 causes the amount of refrigerant at the outlet of the second expansion device 60 to decrease and the amount of refrigerant that flows through the upper heat exchanger 50A to decrease. Consequently, the amount of heat exchanged between the refrigerant and the frost decreases. At this time, as the amount of refrigerant that flows through the indoor heat exchanger 40 increases, the heating capacity increases.

The refrigerant that has condensed at the upper heat exchanger 50A joins, at a joint portion of the refrigerant pipe 86A and the refrigerant pipe 85, with the refrigerant that has condensed at the indoor heat exchanger 40 and whose pressure has been reduced by the first expansion device 30. The resultant refrigerant flows through the refrigerant pipe 86B.

The refrigerant flowing through the refrigerant pipe 86B flows into the lower heat exchanger 50B and evaporates. The refrigerant then flows into the compressor 10 again from the refrigerant pipe 91 through the refrigerant pipe 87B, the second flow switching device 70, the refrigerant pipe 89, and the first flow switching device 20.

If the outdoor heat exchanger 50 is frosted while the normal heating operation is performed and the necessity of defrosting, for example, the lower heat exchanger 50B arises, the second flow switching device 70 is caused to operate such that the hot gas bypass pipe 88 and the refrigerant pipe 87B are connected to each other and the refrigerant pipe 87A and the refrigerant pipe 89 are connected to each other. Consequently, part of the high-temperature and high-pressure gas refrigerant discharged from the compressor 10 flows into the hot gas bypass pipe 80, and the rest of the high-temperature and high-pressure gas refrigerant flows to the indoor heat exchanger 40 through the refrigerant pipe 82, the first flow switching device 20, and the refrigerant pipe 83.

The pressure of the high-temperature and high-pressure gas refrigerant that has flowed into the hot gas bypass pipe 80 is reduced by the second expansion device 60. The refrigerant then flows into the lower heat exchanger 50B, which is the defrosting target, through the hot gas bypass pipe 88, the second flow switching device 70, and the refrigerant pipe 87B. The high-temperature gas refrigerant that has flowed into the lower heat exchanger 506 exchanges heat with frost to condense, and the lower heat exchanger 506 is thus defrosted.

The refrigerant that has condensed at the lower heat exchanger 50B joins, at a joint portion of the refrigerant pipe 86B and the refrigerant pipe 85, with the refrigerant that has condensed at the indoor heat exchanger 40 and whose pressure has been reduced by the first expansion device 30. The resultant refrigerant flows through the refrigerant pipe 86A.

The refrigerant flowing through the refrigerant pipe 86A flows into the upper heat exchanger 50A and evaporates. The refrigerant then flows into the compressor 10 again from the refrigerant pipe 91 through the refrigerant pipe 87A, the second flow switching device 70, the refrigerant pipe 89, and the first flow switching device 20.

As for the defrosting order of the upper heat exchanger 50A and the lower heat exchanger 50B connected in parallel with each other, it is preferable to defrost the lower heat exchanger 50B, then defrost the upper heat exchanger 50A, and then defrost the lower heat exchanger 50B again. The reason for this order will be described below.

For example, the case of defrosting the upper heat exchanger 50A and then defrosting the lower heat exchanger 50B is considered. During defrosting of the upper heat exchanger 50A, frost on the heat transfer fins 52 melts to become water droplets and the water droplets flow downward on the surfaces of the heat transfer fins 52 of the upper heat exchanger 50A. Hereinafter, the water droplets or a stream of water of the melted frost is referred to as drain water. Part of the drain water that has flowed downward from the upper heat exchanger 50A to the lower heat exchanger 50B refreezes on the lower heat exchanger 50B used as an evaporator.

When the lower heat exchanger 50B is subsequently defrosted, it is necessary to defrost frost generated on the heat transfer fins 52 of the lower heat exchanger 50B during the normal heating operation and the drain water that has flowed downward from the upper heat exchanger 50A and has refrozen. Thus, the time required to complete defrosting increases. At this time, as the upper heat exchanger 50A is being used as an evaporator, an amount of frost on the upper heat exchanger 50A increases. Accordingly, when the upper heat exchanger 50A is defrosted next time, the time required to complete defrosting increases.

Therefore, the lower heat exchanger 50B is defrosted first to defrost the frost generated during the normal heating operation, and the upper heat exchanger 50A is defrosted next to defrost the frost generated during the normal heating operation. Lastly, the lower heat exchanger 50B is defrosted again to defrost part of the drain water that has flowed downward from the upper heat exchanger 50A and has refrozen. This can reduce the time for defrosting.

Description will be given next of a problem of the heating-defrosting operation in the refrigerant circuit including the outdoor heat exchanger 50 including the upper heat exchanger 50A and the lower heat exchanger 50B, which are upper and lower divided portions.

FIG. 2 is a schematic diagram illustrating a cross-section of the outdoor heat exchanger 50 and the outdoor fan 500 of an air-conditioning apparatus 100A of the related art. FIG. 3 is a schematic diagram illustrating a cross section of a portion around a boundary between the upper heat exchanger 50A and the lower heat exchanger 50B when the upper heat exchanger 50A of the outdoor heat exchanger 50 of the air-conditioning apparatus 100A of the related art is defrosted. FIG. 4 is a schematic diagram illustrating the cross section of the portion around the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B when the lower heat exchanger 50B of the outdoor heat exchanger 50 of the air-conditioning apparatus 100A of the related art is defrosted. FIG. 5 is a Mollier chart at the time of the heating-defrosting operation of the air-conditioning apparatus 100A of the related art. FIG. 6 is a temperature distribution chart of the portion around the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B at the time of the heating-defrosting operation of the air-conditioning apparatus 100A of the related art.

Broken lines illustrated in FIGS. 2 to 4 represent a boundary line 50C between the upper heat exchanger 50A and the lower heat exchanger 50B. Black arrows illustrated in FIGS. 3 and 4 represent a flow of refrigerant. A thick solid line illustrated in FIG. 6 represents a temperature at each position when the upper heat exchanger 50A is defrosted. A thick broken line illustrated in FIG. 6 represents a temperature at each position when the lower heat exchanger 50B is defrosted. FIG. 6 illustrates a temperature distribution of the hairpin pipes 51 (hereinafter, referred to as boundary-upper-side hairpin pipes 51 a and boundary-lower-side hairpin pipes 51 b) located windward and at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B, and a temperature distribution of the hairpin pipes 51 located leeward is similar to the temperature distribution of the hairpin pipes 51 located windward. The number of steps of the heat transfer pipe, the number of separate heat transfer fins 52, the number of columns of the heat transfer pipe, and the number of paths in the outdoor heat exchanger 50 illustrated in FIGS. 2 to 4 are merely examples, and the configuration is not limited to the configuration of the outdoor heat exchanger 50 illustrated in FIGS. 2 to 4.

In the related art, when the upper heat exchanger 50A disposed above the lower heat exchanger 50B is defrosted, high-temperature refrigerant flows into the upper heat exchanger 50A from the single boundary-upper-side hairpin pipe 51 a as illustrated in FIG. 3. Then, frost on the heat transfer fin 52 of the upper heat exchanger 50A melts, and the resultant drain water flows downward on the surface of the heat transfer fin 52 of the upper heat exchanger 50A and flows onto the surface of the heat transfer fin 52 of the lower heat exchanger 50B. While the upper heat exchanger 50A is defrosted, the lower heat exchanger 50B is used as an evaporator.

When the temperature at the lower heat exchanger 50B decreases to 0 degrees C. or lower, the drain water that has flowed downward from the upper heat exchanger 50A refreezes on the lower heat exchanger 50B.

Subsequently, as illustrated in FIG. 4, the high-temperature refrigerant flows into the lower heat exchanger 50B from the single boundary-lower-side hairpin pipe 51 b to defrost the lower heat exchanger 50B. Consequently, the ice that has refrozen (hereinafter, referred to as refrozen ice) melts. At this time, the upper heat exchanger 50A is used as an evaporator. The temperature does not easily increase at a portion around the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B owing to heat transfer between the upper heat exchanger 50A and the lower heat exchanger 50B. Thus, the refrozen ice at the portion around the boundary grows without melting and disturbs an air flow generated by the outdoor fan 500 to decrease an amount of exchanged heat, leading to a decrease in a heating capacity. In addition, the evaporating temperature of the refrigerant that flows through the evaporator decreases, and an amount of frost on the heat transfer fins 52 of the evaporator increases. The increase in the amount of frost causes problems such as a decreased heating capacity and an increased time required to complete defrosting when the upper heat exchanger 50A is defrosted again.

In the refrigerant passage configuration of the related art, that is, the path configuration as illustrated in FIGS. 2 to 4, the refrigerant that flows through the boundary-upper-side hairpin pipe 51 a and the boundary-lower-side hairpin pipe 51 b when the upper heat exchanger 50A is defrosted as illustrated in FIG. 3 is two-phase refrigerant as illustrated in FIG. 5. In addition, the refrigerant that flows through the boundary-upper-side hairpin pipe 51 a and the boundary-lower-side hairpin pipe 51 b when the lower heat exchanger 50B is defrosted as illustrated in FIG. 4 is two-phase refrigerant as illustrated in FIG. 5. Thus, as illustrated in FIG. 6, a temperature Ta1 at the boundary-lower-side hairpin pipe 51 b when the lower heat exchanger 50B is defrosted is lower than Ta2, described later, in Embodiment 1. In addition, a temperature Tc1 at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B is 0 degrees C. or lower when the upper heat exchanger 50A is defrosted and when the lower heat exchanger 50B is defrosted. That is, a region where the temperature does not exceed 0 degrees C. is present. The refrozen ice does not melt in that region. A temperature Te1 illustrated in FIG. 6 is the temperature at the boundary-lower-side hairpin pipe 51 b when the upper heat exchanger 50A is defrosted.

FIG. 7 is a schematic diagram illustrating a cross section of a portion around a boundary between the upper heat exchanger 50A and the lower heat exchanger 50B when the upper heat exchanger 50A of the outdoor heat exchanger 50 of the air-conditioning apparatus 100 according to Embodiment 1 of the present disclosure is defrosted. FIG. 8 is a schematic diagram illustrating a cross section of the portion around the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B when the lower heat exchanger 50B of the outdoor heat exchanger 50 of the air-conditioning apparatus 100 according to Embodiment 1 of the present disclosure is defrosted. FIG. 9 is a Mollier chart at the time of the heating-defrosting operation of the air-conditioning apparatus 100 according to Embodiment 1 of the present disclosure. FIG. 10 is a temperature distribution chart of the portion around the boundary in the outdoor heat exchanger 50 at the time of the heating-defrosting operation of the air-conditioning apparatus 100 according to Embodiment 1 of the present disclosure.

Broken lines illustrated in FIGS. 7 and 8 represent the boundary line 50C between the upper heat exchanger 50A and the lower heat exchanger 50B. Black arrows illustrated in FIGS. 7 and 8 represent a flow of refrigerant. A thick solid line illustrated in FIG. 10 represents a temperature at each position when the upper heat exchanger 50A is defrosted. A thick broken line illustrated in FIG. 10 represents a temperature at each position when the lower heat exchanger 50B is defrosted. FIG. 10 illustrates a temperature distribution of the boundary-upper-side hairpin pipes 51 a and the boundary-lower-side hairpin pipes 51 b located windward, and a temperature distribution of the boundary-upper-side hairpin pipes 51 a and the boundary-lower-side hairpin pipes 51 b located leeward is similar to the temperature distribution of the boundary-upper-side hairpin pipes 51 a and the boundary-lower-side hairpin pipes 51 b located windward. A temperature Te2 illustrated in FIG. 10 is the temperature at the boundary-lower-side hairpin pipes 51 b when the upper heat exchanger 50A is defrosted. The number of steps of the heat transfer pipe, the number of separate heat transfer fins 52, the number of columns of the heat transfer pipe, and the number of paths in the outdoor heat exchanger 50 illustrated in FIGS. 7 and 8 are merely examples, and the configuration is not limited to the configuration of the outdoor heat exchanger 50 illustrated in FIGS. 7 and 8.

A path configuration of the outdoor heat exchanger 50 of the air-conditioning apparatus 100 according to Embodiment 1 will be described below.

As illustrated in FIG. 7, all the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A, which is the defrosting target, are arranged to be used as inlets for high-temperature gas refrigerant (hereinafter, referred to as refrigerant inlets) at the time of the heating-defrosting operation. Likewise, as illustrated in FIG. 8, all the hairpin pipes 51 located at the uppermost step of the lower heat exchanger 50B, which is the defrosting target, are arranged to be used as refrigerant inlets at the time of the heating-defrosting operation. That is, all the hairpin pipes 51 located at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B are arranged to be used as refrigerant inlets when defrosting is performed.

The hairpin pipes 51 located at the boundary are the hairpin pipes 51 that face the boundary line 50C in the schematic diagram illustrating the cross-section of the outdoor heat exchanger 50 illustrated in FIGS. 7 and 8. Specifically, in the upper heat exchanger 50A, the hairpin pipes 51 located at the boundary are the boundary-upper-side hairpin pipes 51 a that are located at the lowermost steps of the respective columns and are adjacent to the lower heat exchanger 50B. In the lower heat exchanger 50B, the hairpin pipes 51 located at the boundary are the boundary-lower-side hairpin pipes 51 b that are located at the uppermost steps of the respective columns and are adjacent to the upper heat exchanger 50A.

As described above, in Embodiment 1, not the single hairpin pipe 51 but all the hairpin pipes 51 located at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B are arranged to be used as the refrigerant inlets when defrosting is performed. In Embodiment 1, two hairpin pipes 51 (the boundary-upper-side hairpin pipes 51 a) in the upper heat exchanger 50A are arranged to be used as the refrigerant inlets when the upper heat exchanger 50A is the defrosting target. Two hairpin pipes 51 (the boundary-lower-side hairpin pipes 51 b) in the lower heat exchanger 50B are arranged to be used as the refrigerant inlets when the lower heat exchanger 50B is the defrosting target. Note that the number of hairpin pipes 51 used as the refrigerant inlets in each of the upper heat exchanger 50A and the lower heat exchanger 50B is not limited to two. For example, when the number of columns in the outdoor heat exchanger 50 is three, the number of hairpin pipes 51 used as the refrigerant inlets in each of the upper heat exchanger 50A and the lower heat exchanger 50B is three.

With the path configuration illustrated in FIGS. 7 and 8, the high-temperature gas refrigerant that flows from the refrigerant inlets during the heating-defrosting operation as illustrated in FIG. 9 flows through the boundary-upper-side hairpin pipes 51 a or the boundary-lower-side hairpin pipes 51 b located at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B. Thus, the temperatures at the boundary-upper-side hairpin pipes 51 a, the boundary-lower-side hairpin pipes 51 b, and the heat transfer fins 52 easily increase.

The controller 300 controls the second expansion device 60 to maintain the temperature Ta2 (>Ta1) at the boundary-lower-side hairpin pipes 51 b at a high temperature. Specifically, when the temperature Ta2 at the boundary-lower-side hairpin pipes 51 b decreases to a previously set temperature Ta1+α or lower, the controller 300 decreases the opening degree of the second expansion device 60. A temperature difference at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B increases. Consequently, a temperature Tc2 at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B becomes higher than 0 degrees C. when the upper heat exchanger 50A is defrosted and when the lower heat exchanger 50B is defrosted as illustrated in FIG. 10. Thus, the heating-defrosting operation is not an operation in which the temperature becomes 0 degrees C. or lower for both of the upper heat exchanger 50A and the lower heat exchanger 50B. Consequently, drain water around the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B is less likely to refreeze.

As described above, the air-conditioning apparatus 100 according to Embodiment 1 includes a refrigerant circuit in which the compressor 10, the indoor heat exchanger 40, the first expansion device 30, the outdoor heat exchanger 50, and the second flow switching device 70 are sequentially connected to each other by pipes and through which refrigerant circulates. The air-conditioning apparatus 100 also includes the hot gas bypass pipes 80 and 88 coupling the discharge port of the compressor 10 and the second flow switching device 70 to each other, and the controller 300 configured to perform a heating-defrosting operation for alternately defrosting the upper heat exchanger 50A and the lower heat exchanger 50B while performing a normal heating operation. The outdoor heat exchanger 50 includes the plurality of hairpin pipes 51, which are part of a heat transfer pipe. All the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A are used as refrigerant inlets when the upper heat exchanger 50A is defrosted. All the hairpin pipes 51 located at the uppermost step of the lower heat exchanger 50B are used as refrigerant inlets when the lower heat exchanger 50B is defrosted.

In the air-conditioning apparatus 100 according to Embodiment 1, the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A and the uppermost step of the lower heat exchanger 50B, that is, all the hairpin pipes 51 located at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B are used as refrigerant inlets when defrosting is performed. Thus, the drain water at the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B is less likely to refreeze, and a heating capacity is less likely to decrease. Therefore, the highly reliable air-conditioning apparatus 100 is provided.

The air-conditioning apparatus 100 according to Embodiment 1 also includes the second expansion device 60 provided at the hot gas bypass pipes 80 and 88. The air-conditioning apparatus 100 also includes the first boundary-temperature detection device 201 configured to directly or indirectly detect a temperature at the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A, and the second boundary-temperature detection device 202 configured to directly or indirectly detect a temperature at the hairpin pipes 51 located at the uppermost step of the lower heat exchanger 50B. The controller 300 is configured to control the second expansion device 60 such that both the temperature detected by the first boundary-temperature detection device 201 and the temperature detected by the second boundary-temperature detection device 202 are higher than 0 degrees C. during the heating-defrosting operation.

In the air-conditioning apparatus 100 according to Embodiment 1, the second expansion device 60 is controlled, on the basis of the temperature detected by the first boundary-temperature detection device 201 and the temperature detected by the second boundary-temperature detection device 202, such that both the temperature at the hairpin pipes 51 located at the lowermost step of the upper heat exchanger 50A and the temperature at the hairpin pipes 51 located at the uppermost step of the lower heat exchanger 50B are higher than 0 degrees C. during the heating-defrosting operation. Thus, the heating-defrosting operation is not an operation in which the temperature becomes 0 degrees C. or lower for both of the upper heat exchanger 50A and the lower heat exchanger 50B. Consequently, drain water around the boundary between the upper heat exchanger 50A and the lower heat exchanger 50B is less likely to refreeze.

During the heating-defrosting operation, the opening degree of the second expansion device 60, the operation frequency of the compressor 10, and the opening degree of the first expansion device 30 may be changed as required. For example, when an amount of heat exchanged at the indoor heat exchanger 40 during the heating-defrosting operation is desired to be increased, the operation frequency of the compressor 10 may be increased. When the amount of heat exchanged at the indoor heat exchanger 40 is desired to be increased, the opening degree of the second expansion device 60 may be decreased. In this case, as the amount of refrigerant flowing through the hot gas bypass pipe 88 decreases, an amount of heat exchanged at the upper heat exchanger 50A or the lower heat exchanger 50B, which is the defrosting target, decreases. When the temperature of the refrigerant discharged from the compressor 10 is desired to be decreased, the opening degree of the first expansion device 30 may be increased.

Embodiment 2

Embodiment 2 of the present disclosure will be described below. Duplicate of the description of Embodiment 1 will be omitted, and components identical or equivalent to the components in Embodiment 1 are denoted by the same reference signs.

FIG. 11 is a diagram of a refrigerant circuit of an air-conditioning apparatus 100 according to Embodiment 2 of the present disclosure.

In Embodiment 1, the second flow switching device 70 is an integrated valve that selectively switches flows of refrigerant toward the upper heat exchanger 50A and the lower heat exchanger 50B. However, in Embodiment 2, the second flow switching device 70 includes four solenoid valves 70A to 70D as illustrated in FIG. 11. Even when the second flow switching device 70 has such a configuration, advantageous effects similar to those of Embodiment 1 are obtained.

Embodiment 3

Embodiment 3 of the present disclosure will be described below. Duplicate of the description of Embodiment 1 will be omitted, and components identical or equivalent to the components in Embodiment 1 are denoted by the same reference signs.

FIG. 12 is a diagram of a refrigerant circuit of an air-conditioning apparatus 100 according to Embodiment 3 of the present disclosure.

In Embodiment 1, the second flow switching device 70 is an integrated valve that selectively switches flows of refrigerant toward the upper heat exchanger 50A and the lower heat exchanger 50B. However, in Embodiment 3, the second flow switching device 70 includes two three-way valves 600 and 700 as illustrated in FIG. 12. Even when the second flow switching device 70 has such a configuration, advantageous effects similar to those of Embodiment 1 are obtained.

Embodiment 4

Embodiment 4 of the present disclosure will be described below. Duplicate of the description of Embodiment 1 will be omitted, and components identical or equivalent to the components in Embodiment 1 are denoted by the same reference signs.

FIG. 13 is a diagram of a refrigerant circuit of an air-conditioning apparatus 100 according to Embodiment 4 of the present disclosure.

In Embodiment 1, the second flow switching device 70 is an integrated valve that selectively switches flows of refrigerant toward the upper heat exchanger 50A and the lower heat exchanger 50B. However, in Embodiment 4, the second flow switching device 70 includes two three-way valves 600A and 700A as illustrated in FIG. 13. Each of these two three-way valves 600A and 700A is a valve that operates by a differential pressure. To ensure the differential pressure, a check valve 90 is provided between the E port of the first flow switching device 20 and a set of a Q port of the three-way valve 600A and a U port of the three-way valve 700A. Even when the second flow switching device 70 has such a configuration, advantageous effects similar to those of Embodiment 1 are obtained.

REFERENCE SIGNS LIST

1 outdoor unit 2 indoor unit 10 compressor 20 first flow switching device 30 first expansion device 40 indoor heat exchanger 50 outdoor heat exchanger 50A upper heat exchanger 50B lower heat exchanger 50C boundary line 51 hairpin pipe 51 a boundary-upper-side hairpin pipe 51 b boundary-lower-side hairpin pipe 52 heat transfer fin 53 header 60 second expansion device 70 second flow switching device 70A to 70D solenoid valve 80 hot gas bypass pipe 81 to 85 refrigerant pipe 86A refrigerant pipe 86B refrigerant pipe 87A refrigerant pipe 87B refrigerant pipe 88 hot gas bypass pipe 89 refrigerant pipe 90 check valve 91 refrigerant pipe 100 air-conditioning apparatus 100A air-conditioning apparatus 200 outdoor-air-temperature detection device 201 first boundary-temperature detection device 202 second boundary-temperature detection device 300 controller 400 indoor fan 500 outdoor fan 600 three-way valve 600A three-way valve 700 three-way valve 700A three-way valve 

1. An air-conditioning apparatus comprising: a refrigerant circuit in which a compressor, an indoor heat exchanger, a first expansion device, an outdoor heat exchanger, and a flow switching device are sequentially connected to each other by pipes and through which refrigerant circulates, the compressor being configured to compress and discharge the refrigerant, the indoor heat exchanger being configured to allow the refrigerant to exchange heat with indoor air, the first expansion device being configured to reduce pressure of the refrigerant, the outdoor heat exchanger including an upper heat exchanger and a lower heat exchanger having passages in parallel with each other and being configured to allow the refrigerant to exchange heat with outdoor air, the flow switching device being configured to switch flows of the refrigerant toward the upper heat exchanger and the lower heat exchanger; a hot gas bypass pipe coupling a discharge port of the compressor and the flow switching device to each other; and a controller configured to perform a heating-defrosting operation for alternately defrosting the upper heat exchanger and the lower heat exchanger while performing a normal heating operation, the outdoor heat exchanger including a plurality of hairpin pipes, which are part of a heat transfer pipe, when the upper heat exchanger is defrosted, among the plurality of hairpin pipes, all hairpin pipes located at a lowermost step of the upper heat exchanger being used as refrigerant inlets, when the lower heat exchanger is defrosted, among the plurality of hairpin pipes, all hairpin pipes located at an uppermost step of the lower heat exchanger being used as refrigerant inlets.
 2. The air-conditioning apparatus of claim 1, further comprising: a second expansion device provided at the hot gas bypass pipe; a first boundary-temperature detection device configured to directly or indirectly detect a temperature at the hairpin pipes located at the lowermost step of the upper heat exchanger; and a second boundary-temperature detection device configured to directly or indirectly detect a temperature at the hairpin pipes located at the uppermost step of the lower heat exchanger, wherein the controller is configured to control, based on the temperature detected by the first boundary-temperature detection device and the temperature detected by the second boundary-temperature detection device, the second expansion device such that both the temperature at the hairpin pipes located at the lowermost step of the upper heat exchanger and the temperature at the hairpin pipes located at the uppermost step of the lower heat exchanger are higher than 0 degrees C. during the heating-defrosting operation. 